Arrangement of transmembrane domains in adrenergic receptors - Similarity to bacteriorhodopsin

G protein-coupled receptors (GPCRs) have seven hydrophobic domains, which are thought to span the lipid bilayer as alpha helical transmembrane domains (TMDs), The tertiary structure of GPCRs has not been determined; however, molecular models of GPCRs have generally been based on bacteriorhodopsin, which is functionally unrelated to GPCRs but has a similar secondary structure, We sought to examine the validity of using bacteriorhodopsin as a scaffold for GPCR model building by experimentally determining the orientation of the TMDs of adrenergic receptors in the plasma membrane, In separate experiments, three sequential amino acid residues (Leu-310, Leu-311, Asn-312) in TMD VII of the beta(2) adrenoreceptor were mutated to the amino acids found in the homologous domain of the alpha(2) adrenoreceptor (Phe, Phe, Phe), Exchange of Asn-312 and Leu-311 in the beta(2) adrenoreceptor resulted in nonfunctional proteins, most likely due to incompatibility of the introduced bulky phenylalanine side chain with adjacent structural domains in the beta(2) adrenoreceptor. This structural incompatibility was ''repaired'' by replacing the specific beta(2) TMD sequence with an alpha(2) receptor sequence. TMD I and TMD II complemented the Asn-312 --> Phe mutation, and TMD III and TMD VI complemented the Leu-311 --> Phe mutation, These results indicate that TMDs I, II, III, and VI surround TMD VII in a counterclockwise orientation analogous to the orientation of TMDs in bacteriorhodopsin.